There is a great worldwide demand for purified fluids, one of the most commercially important of which is production of fresh water. Many areas of the world have insufficient fresh water for drinking or agricultural uses, and in other areas where plentiful supplies of fresh water exist, the water is often polluted with chemical or biological contaminants, metal ions and the like. There is also a continuing need for commercial purification of other fluids such as industrial chemicals and food juices. U.S. Pat. No. 4,759,850, for example, discusses the use of reverse osmosis for removing alcohols from hydrocarbons in the additional presence of ethers, and U.S. Pat. No. 4,959,237 discusses the use of reverse osmosis for purifying orange juice.
Aside from distillation techniques, purification of water and other fluids is commonly satisfied by filtration. There are many types of filtration, including reverse osmosis (RO), which may involve ultra-filtration or hyper-filtration, and all such technologies are referred to herein using the generic term, “filtration.”
Reverse osmosis involves separation of constituents under pressure using a semi-permeable membrane. As used herein, the term membrane refers to a functional filtering unit, and may include one or more semi-permeable layers and one or more support layers. Depending on the fineness of the membrane employed, reverse osmosis can remove particles varying in size from the macro-molecular to the microscopic, and modern reverse osmosis units are capable of removing particles, bacteria, spores, viruses, and even ions such as Cl− or Ca++.
There are several problems associated with reverse osmosis (RO), including excessive fouling of the membranes and high energy costs associated with producing the required pressure across the membranes. These two problems are interrelated in that most or all of the known RO units require flushing of the membranes during operation with a relatively large amount of feed liquid relative to the amount of permeate produced. The ratio of flushing liquid to permeate recovery in sea water desalination, for example, is about 3:1. Because only a portion of the water being pumped is recovered as purified water, energy used to pump the excess brine is wasted, creating an inherent inefficiency.
It is known to mitigate the energy cost of filtration pumping by employing a work exchange pump such as that described in U.S. Pat. No. 3,489.159 to Cheng et al. (January 1970) which is incorporated herein by reference. In such systems, pressure in the flushing or “waste” fluid that flows past the filter elements is used to pressurize the feed fluid. Unfortunately, known work exchange pumps employ relatively complicated piping, and in any event are discontinuous in their operation. These factors add greatly to the overall cost of installation and operation.
It is also known to mitigate the energy cost of filtration pumping by employing one or more turbines to recover energy contained in the waste fluid. A typical example is included as FIG. 3 in PCT/ES96/00078 to Vanquez-Figueroa (publ. October 1996), which is also incorporated herein by reference. In that example, a feed fluid is pumped up a mountainside, allowed to flow into a filtration unit partway down the mountain, and the waste fluid is run through a turbine to recover some of the pumping energy.
A more generalized schematic of a prior art filtration system employing an energy recovery turbine is shown in FIG. 1. There a filtration system 10 generally comprises a pump 20, a plurality of parallel permeators 30, an energy recovery turbine 40, and a permeate or filtered fluid holding tank 50. The fluid feed lines are straightforward, with an intake line (not shown) carrying a feed fluid from a pretreatment device (not shown) to the pump 20, a feed fluid line 22 conveying pressurized feed fluid from the pump 20 to the permeators 30, a permeate collection line 32 conveying depressurized permeate from the permeators 30 to the holding tank 50, a waste fluid collection line 34 conveying pressurized waste fluid from the permeators 30 to the energy recovery turbine 40, and a waste fluid discharge line 42 conveying depressurized waste fluid from the energy recovery turbine 40 away from the system 10.
A system according to FIG. 1 may be relatively energy efficient, but is still somewhat complicated from a piping standpoint. Among other things, each permeator 30 has at least three pressure connections—one for the feed fluid, one for the waste fluid, and one for the permeate. In a large system such fluid connections may be expensive to maintain, especially where filtration elements in the permeators need to be replaced every few years.
U.S. Pat. No. 547,0469 to Eckman (November 1995) describes a pressure vessel that houses one or more hollow fiber membrane cartridges. The outer circumference of the membranes do not extend completely to the inner wall of the production vessel, allowing convenient replacement of the cartridges, and also providing an annular space between the outer portion of the filters and the inner wall of the production vessel that is used as part of the waste fluid flowpath. The annular space is only continuous along a single cartridge, however, and is interrupted between adjacent cartridges by an annular sealing ring at one end of each cartridge.
WIPO publication 98/46338 discloses an improvement over Eckman in which the annular spaces between the outer portion of the membranes and the inner wall of the production vessel can be continuous past multiple modules (cartridges). Among other things, the improvement extends the convenient replacement benefits of the Eckman design to spiral wound filters.
Both U.S. Pat. No. 547,0469 and WIPO 98/46338 are also advantageous in that they reduce the ratio of couplings relative to the number of filters. In an ordinary reverse osmosis filtration system, three couplings are required to provide fluid flow paths to a single membrane, one coupling for each of the feed fluid, waste fluid, and permeate flow paths. The ratio is thus 3:1. However, in the U.S. Pat. No. 547,0469 and WIPO 98/46338 designs, only three couplings are still required to provide fluid flow paths to multiple membranes. Thus, if the pressure vessel contains three membranes, the ratio is 3:3, and if the pressure vessel contains five membranes, the ratio is 3:5.
It would be advantageous to reduce the ratio of couplings relative to the number of filters still further, but five membranes is usually considered to be the upper limit in an Eckman type system because pressure drops past the several membranes reduce the feed fluid pressure to an undesirable degree. Thus, there is still a need to provide filtration systems, and especially reverse osmosis filtration systems, that reduce the ratio of couplings relative to the number of filters (the coupling/filter ratio) to less than 3:5.